Circulation

Introduction:

The highest tides on earth occur in the Gulf of Maine, ranging as much as 16 meters (52 feet) in the Minas Basin in the easternmost reaches of the Bay of Fundy. Tidal ranges in the Gulf decrease from east to west, but are everywhere greater than the world wide average of about a meter (3 feet). The relatively strong currents created by these tides keep waters well mixed, increasing the availability of nutrients and fueling the regions biological productivity. This connection between tides and biology is best illustrated on Georges Bank, at the southern boundary of the Gulf of Maine, historically one the most productive fishing grounds in the world. Here, strong tidal currents, relatively closed circulation and shallow waters set the stage for dense, protracted phytoplankton blooms (figure 1). These blooms, often observed from space, form the base of a food web that has attracted fishermen for hundreds of years in their pursuit of cod, haddock, halibut, flounder, lobster, scallops, clams and others.

The nutrient rich waters of the Gulf are fed primarily by the cold, deep Labrador Current that enters the along the Scotian Shelf and Northeast Channel to the south of Nova Scotia (figure 2). This current helps drive the primarily counterclockwise circulation of the Gulf, feeding the Eastern Maine Coastal Current (EMCC) off Maine’s east coast and the Western Maine Coastal Current (WMCC) off the coasts of western Maine, New Hampshire and Massachusetts (figure 3). These currents are influenced by fluctuations in river outflow, often enhanced during spring runoff. Water exits the Gulf primarily through the Great South Channel, between Cape Cod and Georges Bank, approximately three months after entering.

The Gulf’s currents have a major impact on biological diversity and abundance through the distribution of nutrients, plankton, larvae, as well as invasive species, toxins and anthropogenic wastes. For example, the recruitment strength of cod on Georges Bank is partly dependent on how tight, or closed, the clockwise circulation is: years with looser circulation results in more off-bank loss of larvae and less recruitment. In another example, variability in the strength and direction of the EMCC influences the occurrence of Paralytic Shellfish Poisoning (PSP) in western Maine. Blooms of the phytoplankton Alexandrium, a source of a neurotoxin responsible for most of the Gulf’s outbreaks of PSP, or Red Tide as it is commonly known, occur naturally in the EMCC. Years when a greater proportion of EMCC continues west instead of turning offshore, results in more delivery of Alexandrium into the inshore waters of western Maine. This can have dire impacts on shellfish harvests as the state is forced to close beds for public safety.

Circulation Maps

Sea surface temperature of Northeast Atlantic, including Gulf of Maine (GoM). Image shows the warm waters of the Gulf Stream and the cold waters of the Labrador Current, which are both loosely labeled. The tidally mixed, cooler waters of the Bay of Fundy and Georges Bank are clearly seen.

The image is a 32 day composite (July 4 – August 4, 2003) from the Aqua-MODIS SST

Major currents of the Northeast Atlantic. Warm currents are colored with reds and colder currents with blues. Gulf of Maine study area is shaded in yellow. Underlying image shows bathymetry with darker blues representing deeper water.

Major currents of the Gulf of Maine. The named portions of the counterclockwise currents are labeled. Underlying image shows bathymetry with darker blues representing deeper water.

Mean 2005 surface current velocity vectors for the Gulf of Maine. Data are from the Gulf of Maine Princeton Ocean Model. The number of data nodes have been subsampled for easier visualization.

Credit: Data from Ocean Modeling Group, School of Marine Sciences, University of Maine.
For more information about the model: http://www.umeoce.maine.edu/
For data: http://rocky.umeoce.maine.edu/las-GoMPOM/main.pl??
Data model output is available through GoMOOS http://www.gomoos.org

Mean 2005 surface current velocity magnitude for the Gulf of Maine. Same data as above, except all available data nodes have been used.

Credit: Data from Ocean Modeling Group, School of Marine Sciences, University of Maine.
For more information about the model: http://www.umeoce.maine.edu/
For data: http://rocky.umeoce.maine.edu/las-GoMPOM/main.pl??

While the gravitational forces of the sun and moon combine to create a continuum of tidal action the world over, it is the unique shape of the Bay of Fundy that contributes to the extraordinary high tides experienced here.

The highest tides of the Bay of Fundy are due to two unique characteristics of this finger of the Gulf. The gradual tapering and shallowing that constricts the tidal flow, causes the waters to rise from an average of one meter (3 feet) found elsewhere in the world to the 16-meter (52 feet) tidal range found at the head of the Bay of Fundy.

The second factor is the precise dimension of this incredible body of water. Every basin of water has its own natural rhythm and at 290 km (180 miles) long, the time it takes for the tide to flood the length of Bay of Fundy is nearly identical to the time it takes for the tide to come in from the adjoining Gulf of Maine. This resonance – the meshing of these two rhythms – means that the tidal range is amplified. Called the "Seiche Effect", this amplification is frequently compared to the wave action produced by a child sloshing water back and forth in a bathtub, each wave higher than the one previous. It is this comparison which led to the Bay of Fundy being called ‘the world’s largest bathtub’.